Short answers, pulled from the story.
Valhalla is a majestic hall located in Asgard, presided over by the god Odin, where warriors slain in battle, known as the Einherjar, live until Ragnarök. The hall is described in the Poetic Edda and Prose Edda as golden and shining, with spear-shaft rafters, shield-thatched roofs, and five hundred and forty doors.
Valhalla derives from the Old Norse Valhǫll, a compound of valr, meaning the slain, and hǫll, meaning hall. Some researchers argue the original meaning was the rock of the slain, referring to an underworld rather than a grand hall, and the word shares an Indo-European root with the modern English word hell.
Every day, the Einherjar dress in war gear, go out to the courtyard, and fight one-on-one combat for sport. Before mealtime they return to feast on the boar Sæhrímnir, which is cooked each day and becomes whole each night, and they drink mead produced by the goat Heiðrún.
Valkyries were battle-maidens employed by Odin to select the most glorious warriors fallen in combat and carry them to Valhalla. They also served ale and mead to the Einherjar during feasts, and the Grímnismál poem names figures including Shaker, Mist, Skeggjöld, Battle-Fetter, and God-Inheritance among those who bore horns of ale to the warriors.
Valhalla is attested in the Poetic Edda, compiled in the 13th century from earlier traditional sources, in the Prose Edda and Heimskringla, both written in the 13th century by Snorri Sturluson, and in the 10th-century anonymous poem Eiríksmál, compiled in Fagrskinna, which commemorates the death of Eric Bloodaxe.
Belief in Valhalla shaped burial customs including the burning of the dead on pyres with their possessions and the burial of horses alongside the deceased. Old Norse sources describe horses, not ships, as the primary means of traveling to Valhalla. In Egils saga, Skallagrímr is buried with his horse, weapons, and smith's tools for this reason.